1. Field of the Invention
This invention relates to limiter circuits and particularly to circuits for improving the linearity of operation of linear circuits that include controllable impedance semiconductor means and circuit means to reduce the conductivity of such semiconductor means to a negligible value when limiting action is not desired.
2. The Prior Art
In known limiter circuits, the impedance of a controllable element connected as the shunt element in a voltage divider is controlled in response to a direct voltage signal obtained by rectifying the input voltage signal to be controlled. Typically, the controllable element is a transistor having its emitter-collector circuit connected in series with another impedance to form the voltage divider. The conductivity of the transistor is controlled by a rectified signal that corresponds to the signal applied across the voltage divider.
In the type of circuit just described, the variation in impedance of the transistor is controlled by a voltage signal. This signal corresponds, in general, to the peak value of the input signal that is to be controlled, or at least is related to the peak value. However, the relationship between the base-emitter voltage V.sub.be and the collector current I.sub.c of a transistor controlled in this way is not linear over a wide range of control voltages. As a result, the operation of the voltage divider will not be linear and the input signal will be attenuated in a non-linear manner and so will be distorted.
In order to overcome this defect, it has heretofore been proposed to provide a different type of circuit for controlling the impedance of the transistor by means of a controlled signal current from a current source instead of a voltage from a voltage source. The current source includes a second transistor that has its collector-emitter circuit connected between a voltage supply terminal and the base of the transistor to be controlled. The magnitude of the control current that passes through the second transistor is controlled by the rectified output voltage signal supplied to the base of the current source transistor.
Even with the improvement afforded by a current controlled limiter circuit, if the limiter circuit is to be made inoperative by switching means, a leakage current will still flow through the collector-emitter circuit of the current source transistor. The leakage current causes the controlling transistor to be very slightly conductive. The controlling transistor is thus in a transitional condition in which its operation is quite non-linear. As a result, the input signal to be controlled will be badly distorted by operation of the limiter circuit.